In the event of overcharging, short circuit, or other such abnormality at a lithium ion battery or other such nonaqueous electrolyte secondary battery, internal temperature rises, in accompaniment to which electrolyte evaporates and internal pressure rises. Nonaqueous electrolyte secondary batteries are therefore provided with explosion prevention valves (safety valves), these being designed so as to allow electrolyte vapor, gaseous decomposition products thereof, and the like to be expelled to the exterior via the explosion prevention valve in the unlikely event that a prescribed internal pressure is exceeded, and various expulsion-preventing contrivances have been developed for preventing expulsion to the exterior of such gases.
As one such expulsion prevention technique, for example, the “secondary battery pack including a secondary battery” described at Patent Reference No. 1 is provided with a filter unit, adsorption by this filter unit of flammable substances expelled from the secondary battery preventing expulsion thereof to the exterior. Furthermore, the “secondary battery” described at Patent Reference No. 2 is provided with a gas-absorbing element including gas-absorbent material at the interior thereof, expulsion of gas and so forth being prevented by suppressing increase in internal pressure of the battery in stable fashion over long periods of time.